Grafting mechanisms in hybrid miniemulsion polymerization

Abstract

ABSTRACT:The ultimate objective of hybrid miniemulsion polymerization is to produce a water-based crosslinkable coating through in situ grafting of a free radical growing acrylic polymer with an unsaturated resin. Certain authors have reported low grafting while others have reported higher. This article explores the factors that influence the grafting tendencies of these systems. Methacrylates such as methyl methacrylate (MMA) have a sterically hindered radical center that lowers its reactivity toward unsaturated resin. This steric hindrance from the methyl group forces grafting of this type of monomer to occur by abstraction of a hydrogen allylic to a resinous double bond. This chain transfer produces a relatively inactive radical on the resin that reduces the grafting efficiency. The transfer process also inherently produces some degree of terminated PMMA polymer within the particle. Grafting occurs in this type of system through termination of living PMMA chains with that radical produced on the resin. For relatively watersoluble monomers such as MMA, grafting efficiency is further lessened by homogeneous nucleation resulting from the monomer hydrophilicity. These newly created particles cannot contain alkyd due to its hydrophobicity and thus inability to transport across the aqueous phase, and hence cannot produce grafted polymer. Nonetheless, degree of grafting of nearly 50% was observed in these systems. For hybrid systems involving an acrylate monomer such as butyl acrylate (BA), virtually complete grafting with alkyd was observed. This is due to the uninhibited BA radical center allowing the molecule to add directly through a resin double bond. This process offers the possibility for complete grafting. Homogeneous nucleation is not involved in this system due to the insolubility of BA in the aqueous phase. Resin double bond content and degree of conjugation also play an integral role in the grafting process.

Waterborne hyperbranched alkyd-acrylic resin obtained by miniemulsion polymerization

“…The properties of these resins were better than alkyd-acrylic blends. In the synthesis of alkyd-acrylic resins, several structures are present by the end of the reaction, which are alkyd resin, acrylic polymer and alkyd-acrylic resin [17,26] .…”

Particle Nucleation Using Different Initiators in the Miniemulsion Polymerization of Styrene

“…Although the former is the most frequently employed, there is an increasing use of oil-soluble initiators. Experimental evidence about the influence of initiation systems in ME polymerization is conflicting and most of the works focus their attention on the polymerization kinetics [Luo and Schork 2002;Tsavalas et al, 2003;Autran et al, 2007;Blythe et al, 1999;Alduncin et al, 1991;Costa et al, 2013].…”

“…This was considered as an indication that the radicals formed in the aqueous phase play an important role in the polymerization. In the ME polymerization of acrylic monomers in the presence of alkyd resins, Tsavalas et al (2003) observed a decrease in the methyl methacrylate (MMA) polymerization rate and the final monomer conversion from the water-soluble initiator (potassium persulfate: KPS) to the oil-soluble initiation system (AIBN or benzoyl peroxide: BPO). It was explained by a reduction in water phase concentration of the oil-phase initiators due to the consumption of a large fraction of the produced radicals in termination reactions within the droplets.…”

Mechanism and Kinetics

“…In the heterogeneous (miniemulsion) grafting of a number of vinyl monomers onto alkyd resins reported by Tsavalas et al, AIBN, BPO, and potassium persulfate (KPS) all show very similar behavior [41] . It is likely that in these cases radicals could effectively be generated only in the aqueous phase, and grow into oligomers through reaction with the monomer in the aqueous phase before entering the hydrophobic alkyd/monomer particles to commence reaction.…”